NASA Technical Reports Server (NTRS) 19910011093: A seven patch hexagonal CP subarray with a CPW/stripline feed network

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by  NASA Technical Reports Server (NTRS)

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NASA Technical Memorandum 103802
A Seven Patch Hexagonal CP
Subarray With a CPW/Stripline
Feed Network
R.N. Simons
Sverdrup Technology, Inc.
Lewis Research Center Group
Brook Park, Ohio
R.Q. Lee and G. R. Lindamood
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio
Prepared for the
1991 North American Radio Science Meeting and
International IEEE/AP-S Symposium
London, Ontario, Canada, June 24-28, 1991
RJASA
A SEVEN PATCH HEXAGONAL CP SUBARRAY WITH
CPW/STRIPLINE FEED NETWORK
R.N. Simons
Sverdrup Technology, Inc.
Lewis Research Center Group
Brook Park, Ohio 44142
R. Q. Lee
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
G.R. Lindamood*
University of Akron
Akron, Ohio 44325
ABSTRACT
A seven element microstrip subarray of hexagonal
geometry has been designed and tested at S-band
frequencies. The microstrip patch antenna is excited at
a single feed position using a direct probe type
connection to generate a circularly polarized (CP) wave.
The RF power is coupled to these feed positions by a
novel coplanar waveguide/stripline feeding network. The
paper describes the subarray architecture and the feed
network in detail. The measured results include the feed
network characterization as well as the radiation
patterns of the subarray.
INTRODUCTION
Microstrip patch antennas have many salient
features such as low cost, light weight, thin profile,
conformability and ease of fabrication which makes them
attractive for satellite communications applications.
These antennas can be excited to radiate linearly
polarized or circularly polarized (CP) waves using only
a single feed. For linear polarization, a rectangular
patch antenna is excited with the feed located near the
center of one edge to achieve proper impedance matching.
To generate CP waves without the use of phase shifters or
hybrid couplers, a nearly square patch must be used with
the feed position determined as given in [1].
This paper describes the architecture of a seven
patch hexagonal CP planar subarray (Fig.l). The seven
patches are fed in equal amplitude and phase by a novel
probe type feed network. The feed is formed by a multi-
layer network which is realized using a combination of
coplanar waveguide (CPW) and balanced stripline.
*Summer Student Intern at NATA Lewis Research Center.
FEED SYSTEM DESIGN AND PERFORMANCE
The feed system consists of a 7-way radial CPW
power divider network [2], a balanced stripline line
stretcher and a CPW to stripline coupler. The RF power to
the radial CPW power divider network (Fig.2) is fed by a
coaxial line. The axis of the coaxial line and the plane
of the CPW circuit are orthogonal. Thus the inner
conductor of the coaxial line meets the junction formed
by the intersecting center strip conductors of the CPW.
The outer conductor of the coaxial line is slotted and
meets the ground planes of the CPW. In this manner, the
seven radial CPW lines are excited in equal amplitude and
phase [2]. The measured return loss at the coaxial input
port, the power coupled to any one of the seven output
ports and the isolation between any two output ports are
shown in Figures 3(a) thru 3(c). At the design frequency
of 2.2875 GHz, the measured insertion loss is only 0.15
dB more than the 8.45 dB theoretically expected for a
seven way divider. The return loss and the isolation are
better than 10 dB and 15 dB respectively.
The output signal from the power divider is
coupled to a balanced stripline line stretcher by a post
coupler. The line stretcher equalizes the path length to
each of the seven patch antennas. Finally, the signal
from the line stretcher is coupled to the patch antennas
by a second post coupler which also works as a probe
feed. Figure 4(a) shows a CPW to a balanced stripline
post coupler. In this coupler, the center strip conductor
of the CPW and the strip conductor of the stripline are
electrically connected by a metal post. The metal post
enters the stripline through an aperture in the ground
plane. The ground plane of the CPW and the stripline are
connected together by bond wires and shorting pins. The
preliminary measured characteristics of the coupler are
presented in Figure 4(b). The insertion loss and return
loss of the coupler are about 0.4 dB and 12 dB at 2.2875
GHz.
SUBARRAY DESIGN AND PERFORMANCE
The patches (4.45 4.39 cros) are arranged in a
X
triangular lattice to form a hexagonal geometry with an
edge to edge spacing of 0.43 inches. The assembled
subarray and the feed and power divider network are shown
in Figs. 5(a) and 5(b) respectively. The measured radia-
0
tion patterns of the array at 2.325 GHz for the = 01
0
and = 901 planes are shown in Figs. 6(a) and 6(b)
respectively. The measured on axis axial ratio for the
LHCP is 1.5 dB and the 3dB beam widths are 36° in both
planes. The gain of the antenna as determined from the
beam widths is 13 dB. The measured return loss at the
coaxial input port of the array is better than 15 dB at
the design frequency and is shown in Fig.7.
2
CONCLUSIONS
The design and performance characteristics of a
CPW/Stripline feed network have been presented. A seven
patch hexagonal CP subarray fed with this feed network
has demonstrated excellent CP patterns.
REFERENCES
1. Y. T. Lo, B. Engst and R. Q. Lee, "Simple Design
Formulas for Circularly Polarized Microstrip Anten-
nas," IEE Proc., Microwaves, Antennas and Propaga-
tion, Vo1.135, Pt.H, No.3, pp.213-215, June 1988.
2. R. N. Simons and G. E. Ponchak, "Coax-to-Channel-
ized Coplanar Waveguide In-Phase N-Way, Radial
Power Divider," Electronics Letters., Vol.26,
No.11, pp.754-755, May 1990.
3
Coaxial line with
Y(4 = 901
Line stretcher slotted outer

1 conductor —7
.-^RF
, input
Tway divider
junction
Coplanar waveguide
power divider
circuit board
` Balanced stripline line
stretcher/coupler circuit
Subarray circuit board
board
Figure II.—Array schematic.
Figure 2.—Radial coax-to-CPW 1:7 power divider.
4
s l i log MAG
REF 0.0 dB
1 10.0 dB/
ad
hp I JO-7 Power divider
—Marker 1
12.2875 GHz
1-6
Start 2.000000000 GHz
Stop 2.500000000 GHz
(a) Measured reflection coefficient at the coaxial input port.
s27 log MAG
REF 0.0 dB
1 10.0 dB/
hp 1 JO-7 Power divider
— Marker 1
12.2875 GHz
1-0
Start 2.000000000 GHz
Stop 2.500000000 GHz
(b)Measured power coupled to output port.
S21 log MAG
REF 0.0 dB
1 10.0 dB/
—15.938 d8
hp 1 JO-7 Power divider
— Marker 1
12.2875 GHz
N
Start 2.000000000 GHz
Stop 2.500000000 GHz
(c)Measured isolation between output ports.
Figure 3.—Coax-to-coplanar waveguide 1:7 power divider.
5
Coplanat:waveguide
input/output (port #1) -1
F- Bond wires
/ f— Stripline ground plarp,
Coplanar s
(rpmmon ground pt"),;
waveguide ^^?^^^ w
w
ground plane --^ /-- Shorting pin
I
T1
Coc r1
/
T2-1 E
Stripline
T2^^
ground
o r2^ plane
C e
o / Ws
Dlel ect t6 5- s-9. , 055 inch
su^b,5,^r-t8 / W = 01 inch
L Stripline WS = 0.1 inch
output/input D1 = 0.05 inch
(port #2) D2 = 0.1 inch
Aperture of diameter / T1 = 0.125 inch
D2 in the stripline ground plane L Metal post of ert = 2.2
diameter, D1 T2 = 0.062 inch
Cr2 = 2.2
(a) CPW-to-stripline post coupler.

S11 log MAG S21 log MAG
Ref 0.0 dB Ref 0.0 dB

A 10.0 dB/ 1 0.5 dB/

1 -12.419 dB/ 0 -0.4018 dB
hp
c
Marker 1
2.2871375 GHz
1
v r:., _, f S21
0
-*- 2
^S11
1
1--io,
Start 0.045000000 GHz
Stop 5.000000000 GHz
(b) Measured coupler characteristics.
Figure 4.
6
(a) Assembled array antenna.
(b) Feed and power divider network.
Figure 5.
0
10
-20
-30
-40
co -50
(a) Along the b = 01 plane.
ai
Q-
aE 0
-10
20
30
-40
-50
-90 -60 -30 0 30 60 90
Angle, degrees
(b) Along the p = 901 plane.
Figure 6.--Measured radiation patterns.
8
m
n -5
ai -10
a
'E
-15
E
m
1 -20
-25
2.22 2.46
Frequency, GHz
Figure 7.—Measured return loss at the coaxial input port.